The present invention relates to a mixer/ amplifier circuit, and more particularly to a mixer/ amplifier circuit for selectively receiving signals in different frequency bands such as VHF and UHF bands, for example, and issuing the signals as the same intermediatefrequency signals substantially at the same level.
Mixer circuits employed in UHF tuner circuits are now constructed of transistors having amplifying capabilities. Signals in the UHF band are amplified at the same time that they are converted into IF signals by the UHF tuner circuit. Therefore, the converted and amplified signals from the UHF tuner circuit and signals received in the VHF band have widely different signal levels when they are selectively received. It has been customary to adjust the bias current for a transistor used as a mixer in the VHF tuner circuit, i.e., as an amplifier in the reception of UHF signals, for varying the amplification factor at the time of receiving signals in the VHF and UHF bands, thus obtaining signals at substantially the same level. To produce the signals at substantially the same level, however, the bias current for the mixer transistor has to be extremely small in the reception of UHF signals, resulting in an increased distortion. In order to keep the distortion below a certain limit, the bias current for the transistor cannot be reduced until the amplification factor is sufficiently lowered. Accordingly, high-level signals have to be issued for the UHF band.
A conventional mixer/amplifier circuit and a circuit arrangement employing such a mixer/amplifier circuit will be described with reference to FIGS. 9 and 10.
As shown in FIG. 10, when a VHF signal is received, a weak VHF signal received by an antenna is supplied to a mixer/amplifier circuit 5 through a VHF circuit 1 composed of a filter 1-1, a tuner 1-2, an RF amplifier 1-3, and a tuner 1-4. When a UHF signal is received, a weak UHF signal received by an antenna is converted into an IF signal by a UHF circuit 3 composed of a tuner 3-1, an RF amplifier 3-2, a tuner 3-3, a mixer 3-4, and an IF filter 3-6, and the IF signal issued from the UHF circuit 3 is supplied to the mixer/amplifier circuit 5. The mixer/amplifier circuit 5 operates in either a first mode to mix the supplied VHF signal with a signal from a VHF local oscillator 2 to produce an IF signal which is supplied to a next IF filter 4, or a second mode to amplify the IF signal converted from the UHF signal and supply the amplified signal to the next IF filter 4.
The mixer/amplifier circuit 5 is illustrated in greater detail in FIG. 9. The first or second mode is selected by applying a power supply voltage as a switching signal (a) or (b) to render a diode D.sub.1 or D.sub.2 conductive. When the UHF signal is received, the VHF local oscillator 2 is separated from the mixer/amplifier circuit 5. In the first mode, the VHF signal supplied from the VHF circuit 1 through the diode D.sub.1 is mixed with the signal from the VHF local oscillator 2, and the mixed signal is applied to the base of a transistor TR.sub.1 for frequency conversion. The signal from the transistor TR.sub.1 is amplified by a transistor TR.sub.2 which supplies the amplified signal to the IF filter 4. In the second mode, the IF signal supplied from the UHF circuit 3 through the diode D.sub.2 is amplified by the transistors TR.sub.1, TR.sub.2, and the amplified signal is supplied to the IF filter 4. As is apparent from FIG. 10, the signal from the UHF circuit 3 is amplified by transistors of the mixer 3-4 in the UHF circuit 3. When matching the output levels of VHF and UHF signals, the foregoing distortion occurs.
As illustrated in FIG. 9, the diodes D.sub.1, D.sub.2 are inserted as switches in series with the signal sources for alternatively selecting the signals from the VHF circuit 1 and the UHF circuit 3. Since the diodes D.sub.1, D.sub.2 have some capacitances between their terminals even when they are not conductive or are reverse-biased, one of the signals tends to be mixed with the other signal while the latter is being received, resulting in signal reception interference, and the tuner 1-4 in the VHF circuit 1 and the IF filter 3-6 in the UHF circuit 3 serve as loads of each other.